Method for making semiconductor device including band-engineered superlattice

ABSTRACT

A method is for making a semiconductor device by forming a superlattice that, in turn, includes a plurality of stacked groups of layers. The method may also include forming regions for causing transport of charge carriers through the superlattice in a parallel direction relative to the stacked groups of layers. Each group of the superlattice may include a plurality of stacked base semiconductor monolayers defining a base semiconductor portion and an energy band-modifying layer thereon. The energy-band modifying layer may include at least one non-semiconductor monolayer constrained within a crystal lattice of adjacent base semiconductor portions so that the superlattice may have a higher charge carrier mobility in the parallel direction than would otherwise occur. The superlattice may also have a common energy band structure therein.

RELATED APPLICATIONS

This application is a continuation of Ser. No. 10/647,061 filed Aug. 22,2003, which is a continuation-in-part of U.S. patent application Ser.Nos. 10/603,696 and 10/603,621 filed on Jun. 26, 2003, the entiredisclosures of which are incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductors, and, moreparticularly, to semiconductors having enhanced properties based uponenergy band engineering and associated methods.

BACKGROUND OF THE INVENTION

Structures and techniques have been proposed to enhance the performanceof semiconductor devices, such as by enhancing the mobility of thecharge carriers. For example, U.S. Patent Application No. 2003/0057416to Currie et al. discloses strained material layers of silicon,silicon-germanium, and relaxed silicon and also including impurity-freezones that would otherwise cause performance degradation. The resultingbiaxial strain in the upper silicon layer alters the carrier mobilitiesenabling higher speed and/or lower power devices. Published U.S. PatentApplication No. 2003/0034529 to Fitzgerald et al. discloses a CMOSinverter also based upon similar strained silicon technology.

U.S. Pat. No. 6,472,685 B2 to Takagi discloses a semiconductor deviceincluding a silicon and carbon layer sandwiched between silicon layersso that the conduction band and valence band of the second silicon layerreceive a tensile strain. Electrons having a smaller effective mass, andwhich have been induced by an electric field applied to the gateelectrode, are confined in the second silicon layer, thus, an n-channelMOSFET is asserted to have a higher mobility.

U.S. Pat. No. 4,937,204 to Ishibashi et al. discloses a superlattice inwhich a plurality of layers, less than eight monolayers, and containinga fraction or a binary compound semiconductor layers, are alternatelyand epitaxially grown. The direction of main current flow isperpendicular to the layers of the superlattice.

U.S. Pat. No. 5,357,119 to Wang et al. discloses a Si-Ge short periodsuperlattice with higher mobility achieved by reducing alloy scatteringin the superlattice. Along these lines, U.S. Pat. No. 5,683,934 toCandelaria discloses an enhanced mobility MOSFET including a channellayer comprising an alloy of silicon and a second materialsubstitutionally present in the silicon lattice at a percentage thatplaces the channel layer under tensile stress.

U.S. Pat. No. 5,216,262 to Tsu discloses a quantum well structurecomprising two barrier regions and a thin epitaxially grownsemiconductor layer sandwiched between the barriers. Each barrier regionconsists of alternate layers of SiO₂/Si with a thickness generally in arange of two to six monolayers. A much thicker section of silicon issandwiched between the barriers.

An article entitled “Phenomena in silicon nanostructure devices” also toTsu and published online Sep. 6, 2000 by Applied Physics and MaterialsScience & Processing, pp. 391-402 discloses a semiconductor-atomicsuperlattice (SAS) of silicon and oxygen. The Si/O superlattice isdisclosed as useful in a silicon quantum and light-emitting devices. Inparticular, a green electromuminescence diode structure was constructedand tested. Current flow in the diode structure is vertical, that is,perpendicular to the layers of the SAS. The disclosed SAS may includesemiconductor layers separated by adsorbed species such as oxygen atoms,and CO molecules. The silicon growth beyond the adsorbed monolayer ofoxygen is described as epitaxial with a fairly low defect density. OneSAS structure included a 1.1 nm thick silicon portion that is abouteight atomic layers of silicon, and another structure had twice thisthickness of silicon. An article to Luo et al. entitled “Chemical Designof Direct-Gap Light-Emitting Silicon” published in Physical ReviewLetters, Vol. 89, No. 7 (Aug. 12, 2002) further discusses the lightemitting SAS structures of Tsu.

Published International Application WO 02/103,767 A1 to Wang, Tsu andLofgren, discloses a barrier building block of thin silicon and oxygen,carbon, nitrogen, phosphorous, antimony, arsenic or hydrogen to therebyreduce current flowing vertically through the lattice more than fourorders of magnitude. The insulating layer/barrier layer allows for lowdefect epitaxial silicon to be deposited next to the insulating layer.

Published-Great Britain Patent Application 2,347,520 to Mears et al.discloses that principles of Aperiodic Photonic Band-Gap (APBG)structures may be adapted for electronic bandgap engineering. Inparticular, the application discloses that material parameters, forexample, the location of band minima, effective mass, etc, can betailored to yield new aperiodic materials with desirable band-structurecharacteristics. Other parameters, such as electrical conductivity,thermal conductivity and dielectric permittivity or magneticpermeability are disclosed as also possible to be designed into thematerial.

Despite considerable efforts at materials engineering to increase themobility of charge carriers in semiconductor devices, there is still aneed for greater improvements. Greater mobility may increase devicespeed and/or reduce device power consumption. With greater mobility,device performance can also be maintained despite the continued shift tosmaller device features.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to provide a method for making a semiconductor devicehaving a higher charge carrier mobility, for example.

This and other objects, features and advantages in accordance with theinvention are provided by a method including forming a superlatticeincluding a plurality of stacked groups of layers, and forming regionsfor causing transport of charge carriers through the superlattice in aparallel direction relative to the stacked groups of layers. Each groupof layers of the superlattice may comprise a plurality of stacked basesemiconductor monolayers defining a base semiconductor portion and anenergy band-modifying layer thereon. Moreover, the energy-band modifyinglayer may comprise at least one non-semiconductor monolayer constrainedwithin a crystal lattice of adjacent base semiconductor portions so thatthe superlattice and has a higher charge carrier mobility in theparallel direction than would otherwise be present. The superlattice mayalso have a common energy band structure therein.

The charge carriers may comprise at least one of electrons and holes. Insome preferred embodiments, each base semiconductor portion may comprisesilicon, and each energy band-modifying layer may comprise oxygen. Eachenergy band-modifying layer may be a single monolayer thick, and eachbase semiconductor portion may be less than eight monolayers thick, suchas two to four layers thick, for example, in some embodiments.

As a result of the band engineering achieved by the present invention,the superlattice may further have a substantially direct energy bandgap.The superlattice may further comprise a base semiconductor cap layer onan uppermost group of layers.

In some embodiments, all of the base semiconductor portions may be asame number of monolayers thick. In other embodiments, at least some ofthe base semiconductor portions may be a different number of monolayersthick. In still other embodiments, all of the base semiconductorportions may be a different number of monolayers thick.

Each non-semiconductor monolayer is desirably thermally stable throughdeposition of a next layer to thereby facilitate manufacturing. Eachbase semiconductor portion may comprise a base semiconductor selectedfrom the group consisting of Group IV semiconductors, Group III-Vsemiconductors, and Group II-VI semiconductors. In addition, each energyband-modifying layer may comprise a non-semiconductor selected from thegroup consisting of oxygen, nitrogen, fluorine, and carbon-oxygen.

The higher charge carrier mobility may result from a lower conductivityeffective mass for the charge carriers in the parallel direction thanwould otherwise be present. The conductivity effective mass may be lessthan two-thirds the conductivity effective mass that would otherwiseoccur. Of course, the superlattice may further comprise at least onetype of conductivity dopant therein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a semiconductor device inaccordance with the present invention.

FIG. 2 is a greatly enlarged schematic cross-sectional view of thesuperlattice as shown in FIG. 1.

FIG. 3 is a perspective schematic atomic diagram of a portion of thesuperlattice shown in FIG. 1.

FIG. 4 is a greatly enlarged schematic cross-sectional view of anotherembodiment of a superlattice that may be used in the device of FIG. 1.

FIG. 5A is a graph of the calculated band structure from the gamma point(G) for both bulk silicon as in the prior art, and for the 4/1 Si/Osuperlattice as shown in FIGS. 1-3.

FIG. 5B is a graph of the calculated band structure from the Z point forboth bulk silicon as in the prior art, and for the 4/1 Si/O superlatticeas shown in FIGS. 1-3.

FIG. 5C is a graph of the calculated band structure from both the gammaand Z points for both bulk silicon as in the prior art, and for the5/1/3/1 Si/O superlattice as shown in FIG. 4.

FIGS. 6A-6H are schematic cross-sectional views of a portion of anothersemiconductor device in accordance with the present invention during themaking thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout and prime notation is used toindicate similar elements in alternate embodiments.

The present invention relates to controlling the properties ofsemiconductor materials at the atomic or molecular level to achieveimproved performance within semiconductor devices. Further, theinvention relates to the identification, creation, and use of improvedmaterials for use in the conduction paths of semiconductor devices.

Applicants theorize, without wishing to be bound thereto, that certainsuperlattices as described herein reduce the effective mass of chargecarriers and that this thereby leads to higher charge carrier mobility.Effective mass is described with various definitions in the literature.As a measure of the improvement in effective mass Applicants use a“conductivity reciprocal effective mass tensor”, M_(e) ⁻¹ and M_(h) ⁻¹for electrons and holes respectively, defined as:${M_{e,{ij}}^{- 1}\left( {E_{F},T} \right)} = \frac{\sum\limits_{E > E_{F}}\quad {\int_{B.Z.}{\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{l}\quad \left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{j}\frac{\partial{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}}{\partial E}{^{3}k}}}}{\sum\limits_{E > E_{F}}{\int_{B.Z.}{{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}\quad {^{3}k}}}}$

for electrons and:${M_{h,{ij}}^{- 1}\left( {E_{F},T} \right)} = \frac{\underset{E < E_{F}}{- \sum}\quad {\int_{B.Z.}{\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{l}\quad \left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{j}\frac{\partial{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}}{\partial E}{^{3}k}}}}{\sum\limits_{E < E_{F}}{\int_{B.Z.}{\left( {1 - {f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}}\quad \right){^{3}k}}}}$

for holes, where f is the Fermi-Dirac distribution, E_(F) is the Fermienergy, T is the temperature, E(k,n) is the energy of an electron in thestate corresponding to wave vector k and the n^(th) energy band, theindices i and j refer to cartesian coordinates x, y and z, the integralsare taken over the Brillouin zone (B.Z.), and the summations are takenover bands with energies above and below the Fermi energy for electronsand holes respectively.

Applicants' definition of the conductivity reciprocal effective masstensor is such that a tensorial component of the conductivity of thematerial is greater for greater values of the corresponding component ofthe conductivity reciprocal effective mass tensor. Again Applicantstheorize without wishing to be bound thereto that the superlatticesdescribed herein set the values of the conductivity reciprocal effectivemass tensor so as to enhance the conductive properties of the material,such as typically for a preferred direction of charge carrier transport.The inverse of the appropriate tensor element is referred to as theconductivity effective mass. In other words, to characterizesemiconductor material structures, the conductivity effective mass forelectrons/holes as described above and calculated in the direction ofintended carrier transport is used to distinguish improved materials.

Using the above-described measures, one can select materials havingimproved band structures for specific purposes. One such example wouldbe a superlattice 25 material for a channel region in a CMOS device. Aplanar MOSFET 20 including the superlattice 25 in accordance with theinvention is now first described with reference to FIG. 1. One skilledin the art, however, will appreciate that the materials identifiedherein could be used in many different types of semiconductor devices,such as discrete devices and/or integrated circuits.

The illustrated MOSFET 20 includes a substrate 21, source/drain regions22, 23, source/drain extensions 26, 27, and a channel regiontherebetween provided by the superlattice 25. Source/drain silicidelayers 30, 31 and source/drain contacts 32, 33 overlie the source/drainregions as will be appreciated by those skilled in the art. Regionsindicated by dashed lines 34, 35 are optional vestigial portions formedoriginally with the superlattice, but thereafter heavily doped. In otherembodiments, these vestigial superlattice regions 34, 35 may not bepresent as will also be appreciated by those skilled in the art. A gate35 illustratively includes a gate insulating layer 37 adjacent thechannel provided by the superlattice 25, and a gate electrode layer 36on the gate insulating layer. Sidewall spacers 40, 41 are also providedin the illustrated MOSFET 20.

Applicants have identified improved materials or structures for thechannel region of the MOSFET 20. More specifically, the Applicants haveidentified materials or structures having energy band structures forwhich the appropriate conductivity effective masses for electrons and/orholes are substantially less than the corresponding values for silicon.

Referring now additionally to FIGS. 2 and 3, the materials or structuresare in the form of a superlattice 25 whose structure is controlled atthe atomic or molecular level and may be formed using known techniquesof atomic or molecular layer deposition. The superlattice 25 includes aplurality of layer groups 45 a-45 n arranged in stacked relation asperhaps best understood with specific reference to the schematiccross-sectional view of FIG. 2.

Each group of layers 45 a-45 n of the superlattice 25 illustrativelyincludes a plurality of stacked base semiconductor monolayers 46defining a respective base semiconductor portion 46 a-46 n and an energyband-modifying layer 50 thereon. The energy band-modifying layers 50 areindicated by stippling in FIG. 2 for clarity of explanation.

The energy-band modifying layer 50 illustratively comprises onenon-semiconductor monolayer constrained within a crystal lattice ofadjacent base semiconductor portions. In other embodiments, more thanone such monolayer may be possible. Applicants theorize without wishingto be bound thereto that energy band-modifying layers 50 and adjacentbase semiconductor portions 46 a-46 n cause the superlattice 25 to havea lower appropriate conductivity effective mass for the charge carriersin the parallel layer direction than would otherwise be present.Considered another way, this parallel direction is orthogonal to thestacking direction. The band modifying layers 50 may also cause thesuperlattice 25 to have a common energy band structure. It is alsotheorized that the semiconductor device, such as the illustrated MOSFET20, enjoys a higher charge carrier mobility based upon the lowerconductivity effective mass than would otherwise be present. In someembodiments, and as a result of the band engineering achieved by thepresent invention, the superlattice 25 may further have a substantiallydirect energy bandgap that may be particularly advantageous foropto-electronic devices, for example, as described in further detailbelow.

As will be appreciated by those skilled in the art, the source/drainregions 22, 23 and gate 35 of the MOSFET 20 may be considered as regionsfor causing the transport of charge carriers through the superlattice ina parallel direction relative to the layers of the stacked groups 45a-45 n. Other such regions are also contemplated by the presentinvention.

The superlattice 25 also illustratively includes a cap layer 52 on anupper layer group 45 n. The cap layer 52 may comprise a plurality ofbase semiconductor monolayers 46. The cap layer 52 may have between 2 to100 monolayers of the base semiconductor, and, more preferably between10 to 50 monolayers.

Each base semiconductor portion 46 a-46 n may comprise a basesemiconductor selected from the group consisting of Group IVsemiconductors, Group III-V semiconductors, and Group II-VIsemiconductors. Of course, the term Group IV semiconductors alsoincludes Group IV—IV semiconductors as will be appreciated by thoseskilled in the art.

Each energy band-modifying layer 50 may comprise a non-semiconductorselected from the group consisting of oxygen, nitrogen, fluorine, andcarbon-oxygen, for example. The non-semiconductor is also desirablythermally stable through deposition of a next layer to therebyfacilitate manufacturing. In other embodiments, the non-semiconductormay be another inorganic or organic element or compound that iscompatible with the given semiconductor processing as will beappreciated by those skilled in the art.

It should be noted that the term monolayer is meant to include a singleatomic layer and also a single molecular layer. It is also noted thatthe energy band-modifying layer 50 provided by a single monolayer isalso meant to include a monolayer wherein not all of the possible sitesare occupied. For example, with particular reference to the atomicdiagram of FIG. 3, a 4/1 repeating structure is illustrated for siliconas the base semiconductor material, and oxygen as the energyband-modifying material. Only half of the possible sites for oxygen areoccupied. In other embodiments and/or with different materials this onehalf occupation would not necessarily be the case as will be appreciatedby those skilled in the art. Indeed it can be seen even in thisschematic diagram, that individual atoms of oxygen in a given monolayerare not precisely aligned along a flat plane as will also be appreciatedby those of skill in the art of atomic deposition.

Silicon and oxygen are currently widely used in conventionalsemiconductor processing, and, hence, manufacturers will be readily ableto use these materials as described herein. Atomic or monolayerdeposition is also now widely used. Accordingly, semiconductor devicesincorporating the superlattice 25 in accordance with the invention maybe readily adopted and implemented as will be appreciated by thoseskilled in the art.

It is theorized without Applicants wishing to be bound thereto, that fora superlattice, such as the Si/O superlattice, for example, that thenumber of silicon monolayers should desirably be seven or less so thatthe energy band of the superlattice is common or relatively uniformthroughout to achieve the desired advantages. The 4/1 repeatingstructure shown in FIGS. 2 and 3, for Si/O has been modeled to indicatean enhanced mobility for electrons and holes in the X direction. Forexample, the calculated conductivity effective mass for electrons(isotropic for bulk silicon) is 0.26 and for the 4/1 SiO superlattice inthe X direction it is 0.12 resulting in a ratio of 0.46. Similarly, thecalculation for holes yields values of 0.36 for bulk silicon and 0.16for the 4/1 Si/O superlattice resulting in a ratio of 0.44.

While such a directionally preferential feature may be desired incertain semiconductor devices, other devices may benefit from a moreuniform increase in mobility in any direction parallel to the groups oflayers. It may also be beneficial to have an increased mobility for bothelectrons or holes, or just one of these types of charge carriers aswill be appreciated by those skilled in the art.

The lower conductivity effective mass for the 4/1 Si/O embodiment of thesuperlattice 25 may be less than two-thirds the conductivity effectivemass than would otherwise occur, and this applies for both electrons andholes. Of course, the superlattice 25 may further comprise at least onetype of conductivity dopant therein as will also be appreciated by thoseskilled in the art.

Indeed, referring now additionally to FIG. 4 another embodiment of asuperlattice 25′ in accordance with the invention having differentproperties is now described. In this embodiment, a repeating pattern of3/1/5/1 is illustrated. More particularly, the lowest base semiconductorportion 46 a′ has three monolayers, and the second lowest basesemiconductor portion 46 b′ has five monolayers. This pattern repeatsthroughout the superlattice 25′ The energy band-modifying layers 50′ mayeach include a single monolayer. For such a superlattice 25′ includingSi/O, the enhancement of charge carrier mobility is independent oforientation in the plane of the layers. Those other elements of FIG. 4not specifically mentioned are similar to those discussed above withreference to FIG. 2 and need no further discussion herein.

In some device embodiments, all of the base semiconductor portions of asuperlattice may be a same number of monolayers thick. In otherembodiments, at least some of the base semiconductor portions may be adifferent number of monolayers thick. In still other embodiments, all ofthe base semiconductor portions may be a different number of monolayersthick.

In FIGS. 5A-5C band structures calculated using Density FunctionalTheory (DFT) are presented. It is well known in the art that DFTunderestimates the absolute value of the bandgap. Hence all bands abovethe gap may be shifted by an appropriate “scissors correction”. Howeverthe shape of the band is known to be much more reliable. The verticalenergy axes should be interpreted in this light.

FIG. 5A shows the calculated band structure from the gamma point (G) forboth bulk silicon (represented by continuous lines) and for the 4/1 Si/Osuperlattice 25 as shown in FIGS. 1-3 (represented by dotted lines). Thedirections refer to the unit cell of the 4/1 Si/O structure and not tothe conventional unit cell of Si, although the (001) direction in thefigure does correspond to the (001) direction of the conventional unitcell of Si, and, hence, shows the expected location of the Si conductionband minimum. The (100) and (010) directions in the figure correspond tothe (110) and (−110) directions of the conventional Si unit cell. Thoseskilled in the art will appreciate that the bands of Si on the figureare folded to represent them on the appropriate reciprocal latticedirections for the 4/1 Si/O structure.

It can be seen that the conduction band minimum for the 4/1 Si/Ostructure is located at the gamma point in contrast to bulk silicon(Si), whereas the valence band minimum occurs at the edge of theBrillouin zone in the (001) direction which we refer to as the Z point.One may also note the greater curvature of the conduction band minimumfor the 4/1 Si/O structure compared to the curvature of the conductionband minimum for Si owing to the band splitting due to the perturbationintroduced by the additional oxygen layer.

FIG. 5B shows the calculated band structure from the Z point for bothbulk silicon (continuous lines) and for the 4/1 Si/O superlattice 25(dotted lines). This figure illustrates the enhanced curvature of thevalence band in the (100) direction.

FIG. 5C shows the calculated band structure from the both the gamma andZ point for both bulk silicon (continuous lines) and for the 5/1/3/1Si/O structure of the superlattice 25′ of FIG. 4 (dotted lines). Due tothe symmetry of the 5/1/3/1 Si/O structure, the calculated bandstructures in the (100) and (010) directions are equivalent. Thus theconductivity effective mass and mobility are expected to be isotropic inthe plane parallel to the layers, i.e. perpendicular to the (001)stacking direction. Note that in the 5/1/3/1 Si/O example the conductionband minimum and the valence band maximum are both at or close to the Zpoint. Although increased curvature is an indication of reducedeffective mass, the appropriate comparison and discrimination may bemade via the conductivity reciprocal effective mass tensor calculation.This leads Applicants to further theorize that the 5/1/3/1 superlattice25′ should be substantially direct bandgap. As will be understood bythose skilled in the art, the appropriate matrix element for opticaltransition is another indicator of the distinction between direct andindirect bandgap behavior.

Referring now additionally to FIGS. 6A-6H, a discussion is provided ofthe formation of a channel region provided by the above-describedsuperlattice 25 in a simplified CMOS fabrication process formanufacturing PMOS and NMOS transistors. The example process begins withan eight-inch wafer of lightly doped P-type or N-type single crystalsilicon with <100> orientation 402. In the example, the formation of twotransistors, one NMOS and one PMOS will be shown. In FIG. 6A, a deepN-well 404 is implanted in the substrate 402 for isolation. In FIG. 6B,N-well and P-well regions 406, 408, respectively, are formed using anSiO₂/Si₃N₄ mask prepared using known techniques. This could entail, forexample, steps of n-well and p-well implantation, strip, drive-in,clean, and re-growth. The strip step refers to removing the mask (inthis case, photoresist and silicon nitride). The drive-in step is usedto locate the dopants at the appropriate depth, assuming theimplantation is lower energy (i.e. 80 keV) rather than higher energy(200-300 keV). A typical drive-in condition would be approximately 9-10hrs. at 1100-1150° C. The drive-in step also anneals out implantationdamage. If the implant is of sufficient energy to put the ions at thecorrect depth then an anneal step follows, which is lower temperatureand shorter. A clean step comes before an oxidation step so as to avoidcontaminating the furnaces with organics, metals, etc. Other known waysor processes for reaching this point may be used as well.

In FIGS. 6C-6H, an NMOS device will be shown in one side 200 and a PMOSdevice will be shown in the other side 400. FIG. 6C depicts shallowtrench isolation in which the wafer is patterned, the trenches 410 areetched (0.3-0.8 um), a thin oxide is grown, the trenches are filled withSiO₂, and then the surface is planarized. FIG. 6D depicts the definitionand deposition of the superlattice of the present invention as thechannel regions 412, 414. An SiO₂ mask (not shown) is formed, asuperlattice of the present invention is deposited using atomic layerdeposition, an epitaxial silicon cap layer is formed, and the surface isplanarized to arrive at the structure of FIG. 6D.

The epitaxial silicon cap layer may have a preferred thickness toprevent superlattice consumption during gate oxide growth, or any othersubsequent oxidations, while at the same time reducing or minimizing thethickness of the silicon cap layer to reduce any parallel path ofconduction with the superlattice. According to the well knownrelationship of consuming approximately 45% of the underlying siliconfor a given oxide grown, the silicon cap layer may be greater than 45%of the grown gate oxide thickness plus a small incremental amount toaccount for manufacturing tolerances known to those skilled in the art.For the present example, and assuming growth of a 25 angstrom gate, onemay use approximately 13-15 angstroms of silicon cap thickness.

FIG. 6E depicts the devices after the gate oxide layers 416 and thegates 418 are formed. To form these layers, a thin gate oxide isdeposited, and steps of poly deposition, patterning, and etching areperformed. Poly deposition refers to low pressure chemical vapordeposition (LPCVD) of silicon onto an oxide (hence it forms apolycrystalline material). The step includes doping with P+ or As− tomake it conducting and the layer is around 250 nm thick.

This step depends on the exact process, so the 250 nm thickness is onlyan example. The pattern step is made up of spinning photoresist, bakingit, exposing it to light (photolithography step), and developing theresist. Usually, the pattern is then transferred to another layer (oxideor nitride) which acts as an etch mask during the etch step. The etchstep typically is a plasma etch (anisotropic, dry etch) that is materialselective (e.g. etches silicon 10 times faster than oxide) and transfersthe lithography pattern into the material of interest.

In FIG. 6F, lowly doped source and drain regions 420, 422 are formedadjacent the channels 424 and 426. These regions are formed using n-typeand p-type LDD implantation, annealing, and cleaning. “LDD” refers ton-type lowly doped drain, or on the source side, p-type lowly dopedsource. This is a low energy/low dose implant that is the same ion typeas the source/drain. An anneal step may be used after the LDDimplantation, but depending on the specific process, it may be omitted.The clean step is a chemical etch to remove metals and organics prior todepositing an oxide layer.

FIG. 6G shows the spacer 428 formation and the source and drainimplants. An SiO₂ mask is deposited and etched back. N-type and p-typeion implantation is used to form the source and drain regions 430, 432,434, and 436. Then the structure is annealed and cleaned. FIG. 6Hdepicts the self-aligned silicides 438 formation, also known assalicidation. The salicidation process includes metal deposition (e.g.Ti), nitrogen annealing, metal etching, and a second annealing. This, ofcourse, is just one example of a process and device in which the presentinvention may be used, and those of skill in the art will understand itsapplication and use in many other processes and devices. In otherprocesses and devices the structures of the present invention may beformed on a portion of a wafer or across substantially all of a wafer.In other processes and devices the structures of the present inventionmay be formed on a portion of a wafer or across substantially all of awafer.

In accordance with another manufacturing process in accordance with theinvention, selective deposition is not used. Instead, a blanket layermay be formed and a masking step may be used to remove material betweendevices, such as using the STI areas as an etch stop. This may use acontrolled deposition over a patterned oxide/Si wafer. The use of anatomic layer deposition tool may also not be needed in some embodiments.For example, the monolayers may be formed using a CVD tool with processconditions compatible with control of monolayers as will be appreciatedby those skilled in the art. Although planarization is discussed above,it may not be needed in some process embodiments. The superlatticestructure may also formed prior to formation of the STI regions tothereby eliminate a masking step. Moreover, in yet other variations, thesuperlattice structure could be formed prior to formation of the wells,for example.

Considered in different terms, the method in accordance with the presentinvention may include forming a superlattice 25 including a plurality ofstacked groups of layers 45 a-45 n. The method may also include formingregions for causing transport of charge carriers through thesuperlattice in a parallel direction relative to the stacked groups oflayers. Each group of layers of the superlattice may comprise aplurality of stacked base semiconductor monolayers defining a basesemiconductor portion and an energy band-modifying layer thereon. Asdescribed herein, the energy-band modifying layer may comprise at leastone non-semiconductor monolayer constrained within a crystal lattice ofadjacent base semiconductor portions so that the superlattice has acommon energy band structure therein, and has a higher charge carriermobility than would otherwise be present.

Other aspects relating to the present invention are disclosed incopending patent applications entitled “SEMICONDUCTOR DEVICE INCLUDINGMOSFET HAVING BAND-ENGINEERED SUPERLATTICE”, and “SEMICONDUCTOR DEVICEINCLUDING BAND-ENGINEERED SUPERLATTICE”, filed concurrently herein, andhaving respective Ser. Nos. 10/647,069, and 10/647,060, the entiredisclosures of which are incorporated herein by reference. In addition,many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

That which is claimed is:
 1. A method for making a semiconductor devicecomprising: forming a superlattice comprising a plurality of stackedgroups of layers; and each group of layers of the superlatticecomprising a plurality of stacked base semiconductor monolayers defininga base semiconductor portion and an energy band-modifying layer thereon;the groups of layers arranged in an alternating pattern of first andsecond groups of layers, with each first group of layers comprisingthree base semiconductor monolayers, and each second group of layerscomprising five base semiconductor monolayers; the energy-band modifyinglayer comprising at least one non-semiconductor monolayer constrainedwithin a crystal lattice of adjacent base semiconductor portions.
 2. Amethod according to claim 1 wherein the superlattice also has a commonenergy band structure therein.
 3. A method according to claim 1 whereinthe superlattice has a higher charge carrier mobility in at least onedirection than would otherwise be present.
 4. A method according toclaim 3 wherein the higher charge carrier mobility results from a lowerconductivity effective mass for the charge carriers in a paralleldirection than would otherwise be present.
 5. A method according toclaim 4 wherein the lower conductivity effective mass is less thantwo-thirds the conductivity effective mass that would otherwise occur.6. A method according to claim 3 wherein the charge carriers having thehigher mobility comprise at least one of electrons and holes.
 7. Amethod according to claim 1 wherein each base semiconductor portioncomprises silicon.
 8. A method according to claim 1 wherein each energyband-modifying layer comprises oxygen.
 9. A method according to claim 1wherein each energy band-modifying layer is a single monolayer thick.10. A method according to claim 1 wherein the superlattice further has asubstantially direct energy bandgap.
 11. A method according to claim 1wherein the superlattice further comprises a base semiconductor caplayer on an uppermost group of layers.
 12. A method according to claim 1wherein each non-semiconductor monolayer is thermally stable throughdeposition of a next layer.
 13. A method according to claim 1 whereineach base semiconductor portion comprises a base semiconductor selectedfrom the group consisting of Group IV semiconductors, Group III-Vsemiconductors, and Group II-VI semiconductors.
 14. A method accordingto claim 1 wherein each energy band-modifying layer comprises anon-semiconductor selected from the group consisting of oxygen,nitrogen, fluorine, and carbon-oxygen.
 15. A method according to claim 1wherein forming the superlattice comprises forming the superlattice on asubstrate.
 16. A method according to claim 1 further comprising dopingthe superlattice with at least one type of conductivity dopant therein.17. A method according to claim 1 wherein the superlattice defines achannel for a transistor.
 18. A method for making a semiconductor devicecomprising: forming a superlattice comprising a plurality of stackedgroups of layers; and each group of layers of the superlatticecomprising a plurality of stacked silicon atomic layers defining asilicon portion and an energy band-modifying layer thereon; the groupsof layers arranged in an alternating pattern of first and second groupsof layers, with each first group of layers comprising three base siliconmonolayers, and each second group of layers comprising five base siliconmonolayers; the energy-band modifying layer comprising at least oneoxygen atomic layer constrained within a crystal lattice of adjacentsilicon portions.
 19. A method according to claim 18 wherein thesuperlattice has a common energy band structure therein.
 20. A methodaccording to claim 18 wherein the superlattice has a higher chargecarrier mobility in at least one direction than would otherwise bepresent.
 21. A method according to claim 20 wherein the higher chargecarrier mobility results from a lower conductivity effective mass forthe charge carriers in a parallel direction than would otherwise bepresent.
 22. A method according to claim 20 wherein the charge carriershaving the higher mobility comprise at least one of electrons and holes.23. A method according to claim 18 wherein each energy band-modifyinglayer is a single atomic layer thick.
 24. A method according to claim 18wherein the superlattice further has a substantially direct energybandgap.
 25. A method according to claim 18 wherein the superlatticefurther comprises a silicon cap layer on an uppermost group of layers.26. A method according to claim 18 wherein forming the superlatticecomprises forming the superlattice on a substrate.
 27. A methodaccording to claim 18 further comprising doping the superlattice with atleast one type of conductivity dopant therein.
 28. A method according toclaim 18 wherein the superlattice defines a channel for a transistor.